1. Field of the Invention
This invention relates to the location of faults in a buried cable and, more specifically, to a method and apparatus for the location of cable splices and cable faults.
2. Discussion of Related Art
Various methods are utilized in practice to locate cable splices and cable faults. There are proven methods to locate cable splices, for example location with the twist method and the location with the minimum distortion method.
For location according to the twist method, both poles of a generator operating at an audio frequency are connected to two inside conductors at the near end of a cable. This arrangement generates a field along the cable route, the so-called twist field, that, in magnitude, reaches a minimum at a point directly above the cable when both inside conductors lie next to each other horizontally and reaches a maximum directly above the cable when both inside conductors lie vertically above each other.
With a receiver that is tuned to the audio frequency of the generator, the route of the cable can be traced. If the twist of the inside conductors is even, i.e., in regular intervals along the cable run, the change between minimum and maximum fields over the cable can be located. If the twist is interrupted, for example by a cable splice, this regularity of the changing twist field is disturbed.
The disadvantage of the twist method is that location of the pitch (i.e., the distance between successive maxima or minima) of the twist field is only possible if the pitch of layers (i.e., the distance between successive occurrences of the two cables in a particular orientation with respect to one another) of the twisted cable of the inside conductors is approximately the same as the distance of the cable to the receiver. For example, if the twist field results from a medium voltage cable with a pitch of layers of a meter, then the cable can be monitored if it is at a depth (the laying depth) of up to approximately one meter. The twist field of a telecommunication cable whose pitch of layers of the twist is only a couple of centimeters cannot be located at a laying depth of one meter. At such a large disparity between the depth and the pitch of layers, the minima and maxima of the field magnitude at the receiver due to the twist field becomes difficult, if not impossible, to resolve.
In another method of locating faults, the so-called minimum distortion method, one pole of a frequency generator is connected to one end of the cable and the other pole is connected to the earth. The end of the cable is also connected to the earth so that the electric circuit for the audio frequency signal is closed. With a receiver that is tuned to the audio frequency of the frequency generator, the cable route can be tracked according to the traditional minimum method. Above the cable route, a homogeneous magnetic field is generated that creates a very sharp minimum in the receiver when the receiver is located over the cable. In the area of a cable splice or a cable fault, the homogeneity of the magnetic field is distorted in a way that is recognizable. The sharpness of the minimum appears diminished as the receiver is passed over the cable.
The disadvantage in this method is that the evaluation of the minimum distortion is very subjective. The user of such a system is required to have considerable experience in locating such faults with the method and to have some technical qualifications. Most operators of such a system, however, cannot be expected to be so qualified.
An improvement for the location of cable faults with the minimum distortion method has been described in U.S. Pat. No. 5,714,885 (the '885 patent). The '885 patent describes a system where the audio frequency generator couples a combination of two frequencies to the cable under test. The receiver locates, via antenna coils, the magnetic field and, via rods inserted into the ground, the electrical field. Evaluating differential field components from two different driving frequencies can definitely improve location of the cable and location of cable faults and cable splices with the receiver. The system and method described in the '885 patent requires comparatively high effort for the creation of the frequencies in the generator and the evaluation of all signal components in the receiver is relatively elaborate.
Another method for location of cable faults is described in German patent DE-PS 19824157 (the '157 patent). As described in the '157 patent, the generator creates two high frequency signals with audio frequency modulation that are coupled into the cable under test. A receiver evaluates these signals and displays the fault by different sounds. The disadvantage of this method is that by using high frequency signals the range is comparatively small and the system is applicable only for sheathed cables, i.e., coaxial cables. Blind laid cable splices cannot be located with this method.
Yet another method to locate cable faults, especially to locate metallic blank insulation faults in ground cables and pipelines, is described in U.S. Pat. No. 5,828,219 (the '219 patent). As described in the '219 patent, a generator couples a very low frequency signal <10 Hz and an audio frequency signal >100 Hz into the conductor under test (e.g., the cable). The receiver includes a combination of magnetometer and antenna coils. Utilizing the combination of detectors in the receiver, a record of the laying depth and the current intensity along the cable route can be produced. Faults can be located by evaluating the gradient between the available test values for the current intensities.
However, in the method described in the '219 patent, it is necessary to galvanically connect the generator directly to the cable to effectively couple the low frequency component to the cable. The large amount of apparative effort from the receiver required, in consequence of the combination of magnetometer and conventional antenna coils, is disadvantageous. Blind laid cable splices also cannot be located with this method.
All known methods and systems currently utilized have the disadvantage that there are high expectations concerning the apparative effort or the evaluation of the complex signals. Analysis of the signals r can be very subjective and requires the user of the receiver to be very experienced and have high technical qualifications. Therefore, there is a need for a system and method for locating cable splices and cable faults that are both and easy to use.